The present invention relates to voice-switching devices. More particularly, the invention relates to voice-switching devices for use in a telephone terminal.
It is well known to enhance the capabilities of a standard telephone terminal by adding a loudspeaker to the receive path of said terminal. However, due to the increased gain requirements in a loudspeaking telephone, a voice-switching device must be incorporated in order to avoid instability. Instability can occur in a terminal where there is strong coupling at the network interface point (e.g. four-to-two wire hybrid) and/or in the loudspeaker-to-microphone path.
Most modem voice-switching devices have three associated operational modes: active receive mode, active transmit mode, and quiescent mode. In active receive mode, a speech signal level on the receive path exceeds those on the transmit path. Similarly, in active transmit mode a speech signal level on the transmit path exceeds those on the receive path. In quiescent mode, no significant speech signals are present on either the transmit or receive path. While in active transmit mode, loss is inserted in the receive channel and removed from the transmit channel. Loss is inserted and removed by varying the amount of attenuation applied to the signal. Conversely, in active receive mode, loss is inserted in the transmit channel and removed from the receive channel. In the quiescent mode, the loss is approximately halved between the transmit and receive channels. The loss which is moved from channel to channel is referred to as xe2x80x9cswitched-lossxe2x80x9d. A voice-switching circuit increases the stability margin of the terminal by ensuring some amount of loss exists in either the transmit or receive or both channels at all times.
Prior art speakerphone arrangements (e.g., group listening terminals) have merely applied the same amount of switched loss to all of the output paths. Such a device would include a single receive variolosser (e.g. a programmable attenuator) which would attenuate all of the attached receivers identically. A disadvantage of such an arrangement is that the amount of switched loss which must be applied to the loudspeaker is greater than that which is necessary for the handset receiverxe2x80x94since less acoustic coupling occurs between the handset receiver and microphone than between a loudspeaker and microphone. This results in the handset receiver being excessively attenuated when the terminal is in transmit mode, i.e. it is more difficult for the handset user to hear the far end talker. The discomfort associated with the excessive attenuation is that either party believes the connection has been lost or has difficulty communicating during double-talk conditions (both parties talking simultaneously). Excessive attenuation also leads to increased noise contrast i.e. noise is more easily heard as it increases and decreases due to voice-switching. Furthermore, since the transition time from active (transmit or receive) to quiescent mode (no speech signals present) or from quiescent to active mode is greater for the loudspeaker than the handset speaker, prior art arrangements result in slower than necessary transition times for the handset speaker. This causes discomfort both parties by cutting off the initial speech signal.
It is an object of the present invention to provide a voice-switching device for applying the optimum amount of switched loss to a plurality of distinct receive transducers independently. A distinct receive transducer is a receive transducer which requires a different amount of switched loss than the other receiver transducer.
According to one aspect of the invention this objective is met by attenuating each output path separately. An output path is herein defined as a receive path to which a plurality of identical receive transducers can be attached. This distinction is made because there is no advantage to applying the same level of switched loss independently to a set of identical receive transducers.
One application of said voice-switching device is for use in a group listening call wherein two distinct receive transducers, e.g. a loudspeaker and a handset speaker, are employed.
According to one embodiment, separate variolossers (attenuators) are attached to the transmit path and the output paths. In response to said device entering into the transmit mode of operation, the switched loss for each output path is calculated and applied independently of the other output paths. Conversely, in response to said device entering into a receive mode of operation, or a quiescent mode of operation, the switched loss for the transmit path is calculated and applied based on the characteristics of the dominant receive transducer. The dominant receive transducer is the receive transducer (or group of identical receive transducers) which provides the greatest amount of coupling to the transmit path.
Ideally this invention will result in the minimum switched loss being applied to each receive transducer thereby providing optimum audio quality and minimal transition times.
One aspect of the present invention provides for a voice-switching device to improve audio quality in a telephone terminal comprising: a transmit path; a receive path; a plurality of output paths; means for sampling the transmit and receive paths in order to identify the presence and magnitude of speech signals; means for determining the mode of operation responsive to sampling the transmit and receive paths; and means for adjusting the amount of switched loss applied to the transmit path and output paths responsive to determining the mode of operation.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.